This invention relates generally to ultrathin polymeric membranes. More specifically, the invention relates to hydrophobic fluorinated polyelectrolyte multilayer films and methods for using them.
Hydrophobic, or water-repelling, coatings are known to be useful for many applications. Fluorinated materials are known for their hydrophobicity. Thus, fluorinated hydrocarbons have been applied as surface treatment to fabric, yarn, upholstery, garments, and carpets to impart water- and stain-repellency to these materials. Well known fluorinated materials include Scotchguard™ Protector (3M Company) and Teflon™ Advanced (DuPont Inc.) carpet protector. Many water-repellant fluorinated materials are anionic perfluorinated small molecules which may be rubbed off surfaces to which they are applied, and which have caused concerns regarding environmental and health effects (in particular, perfluorooctanesulfonates). There is a need for a persistent fluorinated coating which has mechanical integrity, such as a thin polymer film.
The term “perfluorinated” as applied to polymers generally refers to polymers comprising repeat units that bear a plurality of fluorine groups (at least two). Unfortunately, fluorinated polymers are hard to process into thin films, as many are insoluble and/or infusible. Thus, coatings of perfluorinated polymers such as polytetrafluoroethylene (PTFE, or Teflon™) are produced by coating an article with PTFE powder then heat-treating to sinter and bind the particles together. Alternatively, a fluorinated polymer film is prepared on a surface by microwave, plasma, or reactive gas deposition of fluorinated monomer. There is a need for materials and methods to produce thin highly fluorinated polymer coatings on surfaces under ambient conditions.
Polyelectrolytes are macromolecules comprising a plurality of charged repeat units. Amorphous complexes may be formed by contacting solutions of polyelectrolytes bearing opposite charges. The driving force for association, or complexation, of polyelectrolytes is multiple ion pairing between oppositely charged repeat units on different molecules.
Recently, thin films of polyelectrolyte complexes have been prepared using polyelectrolytes which are alternately deposited on a substrate or substratum. See Decher and Schlenoff, Eds., Multilayer Thin Films—Sequential Assembly of Nanocomposite Materials, Wiley-VCH, Weinheim (2003); Decher, Science, 277, 1232 (1997). Decher and Hong (U.S. Pat. No. 5,208,111) disclose a method for a buildup of multilayers by alternating dipping, i.e., cycling a substrate between two reservoirs containing aqueous solutions of polyelectrolytes of opposite charge, with an optional rinse step in polymer-free solution following each immersion. Each cycle adds a layer of polymer via ion pairing forces to the oppositely-charged surface and reverses the surface charge thereby priming the film for the addition of the next layer. Films prepared in this manner tend to be uniform, follow the contours and irregularities of the substrate, and are typically between about 10 nm and about 10,000 nm thick. The thickness of a film depends on many factors, including the number of layers deposited, the ionic strength of the solutions, the types of polymers, the deposition time, the solution pH, the temperature, and the solvent used. Although studies have shown that the substantial interpenetration of the individual polymer layers results in little composition variation over the thickness of a film, such polymer thin films are, nevertheless, referred to as polyelectrolyte multilayers (PEMUs).